Nervous system

Organization of the Nervous System
With a mass of only 2 kg (4.5 lb), about 3% of the total body
weight, the nervous system is one of the smallest and yet the
most complex of the 11 body systems.

Central Nervous System
• The central nervous system (CNS) consists of the brain and
spinal cord .
• The brain is the part of the CNS that is located in the skull and
contains about 85 billion neurons.
• The spinal cord is connected to the brain through the foramen
magnum of the occipital bone and is encircled by the bones of
the vertebral column.
• The spinal cord contains about 100 million neurons. The CNS
processes many different kinds of incoming sensory
information.
• It is also the source of thoughts, emotions, and memories. Most
signals that stimulate muscles to contract and glands to secrete
originate in the CNS.

Peripheral Nervous System
• The peripheral nervous system (PNS) consists of all nervous
tissue outside the CNS.
• Components of the PNS include nerves, ganglia, enteric
plexuses, and sensory receptors.
• Twelve pairs of cranial nerves emerge from the brain and
thirty-one pairs of spinal nerves emerge from the spinal cord.
Each nerve follows a defined path and serves a specific region
of the body

• Ganglia
(GANG-gle¯-a swelling or knot; singular is ganglion) are small
masses of nervous tissue, consisting primarily of neuron cell
bodies, that are located outside of the brain and spinal cord.
Ganglia are closely associated with cranial and spinal nerves

• Enteric plexuses :- (PLEK-sus-ez) are extensive networks of
neurons located in the walls of organs of the gastrointestinal
tract.
• The neurons of these plexuses help regulate the digestive
system

• The PNS is divided into a somatic nervous system (SNS)
(soma body), an autonomic nervous system (ANS) (auto-
self; -nomic law), and an enteric nervous system (ENS)
(enteron intestines).
• The SNS consists of
(1) sensory neurons that convey information to the CNS from
somatic receptors in the head, body wall, and limbs and from
receptors for the special senses of vision, hearing, taste, and
smell.
(2) motor neurons that conduct impulses from the CNS to
skeletal muscles only. Because these motor responses can be
consciously controlled, the action of this part of the PNS is
voluntary.

FUNCTIONS OF ANS
• The ANS consists of
(1) sensory neurons that convey information to the CNS
from autonomic sensory receptors, located primarily in
visceral organs such as the stomach and lungs,
(2) motor neurons that conduct nerve impulses from the
CNS to smooth muscle, cardiac muscle, and glands.
Because its motor responses are not normally under
conscious control, the action of the ANS is involuntary.
• The motor part of the ANS consists of two branches, the
sympathetic division and the parasympathetic
division.

FUNCTIONS OF ENS
• The operation of the ENS, the “brain of the gut,” is involuntary.
Once considered part of the ANS, the ENS consists of over 100
million neurons in enteric plexuses that extend most of the
length of the gastrointestinal (GI) tract.
• Many of the neurons of the enteric plexuses function
independently of the ANS and CNS to some extent, although
they also communicate with the CNS via sympathetic and
parasympathetic neurons.

• Sensory neurons of the ENS monitor chemical changes within
the GI tract as well as the stretching of its walls.
• Enteric motor neurons govern contractions of GI tract smooth
muscle to propel food through the GI tract, secretions of GI tract
organs (such as acid from the stomach), and activities of GI
tract endocrine cells, which secrete hormones.

Functions of the Nervous System
• The nervous system carries out a complex array of tasks. It allows
us
to sense various smells,
 produce speech, and remember past events
 in addition, it provides signals that control body movements and
regulates the operation of internal organs.
These diverse activities can be grouped into three basic functions:
1. sensory(input),
2. integrative (process),
3. and motor (output).

Sensory function
• Sensory receptors detect internal stimuli, such as an increase in
blood pressure, or external stimuli (for example, a raindrop
landing on your arm).
• This sensory information is then carried into the brain and spinal
cord through cranial and spinal nerves.

Integrative function.
• The nervous system processes sensory information by analyzing it
and making decisions for appropriate responses an activity known
as integration

Motor function
• Once sensory information is integrated, the nervous system
may elicit an appropriate motor response by activating
effectors (muscles and glands) through cranial and spinal
nerves.
• Stimulation of the effectors causes muscles to contract and
glands to secrete.

Signal Transmission at Synapses
• Synapse is a region where communication occurs between two
neurons or between a neuron and an effector cell (muscle cell
or glandular cell).
• The term presynaptic neuron refers to a nerve cell that
carries a nerve impulse toward a synapse. It is the cell that
sends a signal.
• A postsynaptic cell is the cell that receives a signal. It may be
a nerve cell called a postsynaptic neuron that carries a nerve
impulse away from a synapse or an effector cell that responds
to the impulse at the synapse.

• Most synapses between neurons are axodendritic (from axon
to dendrite), while others are axosomatic (from axon to cell
body) or axoaxonic(from axon to axon).
• In addition, synapses may be electrical or chemical and they
differ both structurally and functionally.

Electrical Synapses
• At an electrical synapse, action potentials (impulses) conduct
directly between the plasma membranes of adjacent neurons
through structures called gap junctions.
• Each gap junction contains a hundred or so tubular connexons,
which act like tunnels to connect the cytosol of the two cells
directly
• As ions flow from one cell to the next through the connexons,
the action potential spreads from cell to cell.
• Gap junctions are common in visceral smooth muscle, cardiac
muscle, and the developing embryo. They also occur in the
brain.

Electrical synapses have two main
advantages
1. Faster communication. Because action potentials conduct directly
through gap junctions, electrical synapses are faster than chemical
synapses.
• At an electrical synapse, the action potential passes directly from the
presynaptic cell to the postsynaptic cell.
• The events that occur at a chemical synapse take some time and
delay communication slightly.

2. Synchronization.
• Electrical synapses can synchronize (coordinate) the activity of a
group of neurons or muscle fibers. In other words, a large
number of neurons or muscle fibers can produce action
potentials in unison if they are connected by gap junctions.
• The value of synchronized action potentials in the heart or in
visceral smooth muscle is coordinated contraction of these fibers
to produce a heartbeat or move food through the
gastrointestinal tract.

Chemical Synapses
Although the plasma membranes of presynaptic and postsynaptic neurons in a
chemical synapse are close, they do not touch.
• They are separated by the synaptic cleft, a space of 20–50 nm* that is filled with
interstitial fluid. Nerve impulses cannot conduct across the synaptic cleft, so an
alternative, indirect form of communication occurs.
• In response to a nerve impulse, the presynaptic neuron releases a neurotransmitter
that diffuses through the fluid in the synaptic cleft and binds to receptors in the
plasma membrane of the postsynaptic neuron. The postsynaptic neuron receives
the chemical signal and in turn produces a postsynaptic potential, a type of graded
potential.

• Thus, the presynaptic neuron converts an electrical signal (nerve
impulse) into a chemical signal (released neurotransmitter). The
postsynaptic neuron receives the chemical signal and in turn
generates an electrical signal (postsynaptic potential).
• The time required for these processes at a chemical synapse, a
synaptic delay of about 0.5 msec, is the reason that chemical
synapses relay signals more slowly than electrical synapses.

A typical chemical synapse transmits a
signal as follows
SSTEP - 1
A nerve impulse arrives at a synaptic end bulb (or at a varicosity) of
a presynaptic axon.
STEP - 2
The depolarizing phase of the nerve impulse opens voltage gated
Ca2 channels, which are present in the membrane of synaptic end
bulbs. Because calcium ions are more concentrated in the
extracellular fluid, Ca2 flows inward through the opened channels.

STEP – 3
• An increase in the concentration of Ca2 inside the presynaptic
neuron serves as a signal that triggers exocytosis of the
synaptic vesicles.
• As vesicle membranes merge with the plasma membrane,
neurotransmitter molecules within the vesicles are released into
the synaptic cleft. Each synaptic vesicle contains several
thousand molecules of neurotransmitter

STEP - 4
• The neurotransmitter molecules diffuse across the synaptic cleft and
bind to neurotransmitter receptors in the postsynaptic neuron’s
plasma membrane

STEP - 5
• Binding of neurotransmitter molecules to their receptors on ligand-
gated channels opens the channels and allows particular ions to flow
across the membrane.
STEP - 6
As ions flow through the opened channels, the voltage across the
membrane changes. This change in membrane voltage is a postsynaptic
potential. Depending on which ions the channels admit, the
postsynaptic potential may be a depolarization (excitation) or a
hyperpolarization (inhibition).

Step – 7
• When a depolarizing postsynaptic potential reaches
threshold, it triggers an action potential in the axon of
the postsynaptic neuron.

Electrical Signals In Neurons
• Like muscle fibers, neurons are electrically excitable. They
communicate with one another using two types of electrical
signals:
(1) Graded potentials are used for short- distance
communication only.
(2) Action potentials allow communication over long distances
within the body. When an action potential occurs in a neuron
(nerve cell), it is called a nerve action potential (nerve
impulse). To understand the functions of graded potentials
and action potentials
• Lets consider how the nervous system allows you to feel the
smooth surface of a pen that you have picked up from a table

1. As you touch the pen, a graded potential develops in a sensory
receptor in the skin of the fingers.
2. The graded potential triggers the axon of the sensory neuron to
form a nerve action potential, which travels along the axon into the
CNS and ultimately causes the release of neurotransmitter at a
synapse with an interneuron.
3. The neurotransmitter stimulates the interneuron to form a graded
potential in its dendrites and cell body.
4. In response to the graded potential, the axon of the interneuron
forms a nerve action potential. The nerve action potential travels
along the axon, which results in neurotransmitter release at the
next synapse with another interneuron.

5. This process of neurotransmitter release at a synapse followed by
the formation of a graded potential and then a nerve action potential
occurs over and over as interneurons in higher parts of the brain (such
as the thalamus and cerebral cortex) are activated.
Once interneurons in the cerebral cortex, the outer part of the brain,
are activated, perception occurs and you are able to feel the smooth
surface of the pen touch your fingers

Suppose that you want to use the pen to write
a letter. The nervous system would respond
in the following way :-
6. A stimulus in the brain causes a graded potential to form in the dendrites
and cell body of an upper motor neuron (a type of motor neuron that
synapses with a lower motor neuron farther down in the CNS in order to
contract a skeletal muscle).
• The graded potential subsequently causes a nerve action potential to occur
in the axon of the upper motor neuron, followed by neurotransmitter
release.

7. The neurotransmitter generates a graded potential in a lower
motor neuron, a type of motor neuron that directly supplies skeletal
muscle fibers.
The graded potential triggers the formation of a nerve action
potential and then release of the neurotransmitter at neuromuscular
junctions formed with skeletal muscle fibers that control
movements of the fingers.

8.The neurotransmitter stimulates the muscle fibers that
control finger movements to form muscle action
potentials. The muscle action potentials cause these
muscle fibers to contract, which allows you to write with
the pen.

• The production of graded potentials and action potentials depends on
two basic features of the plasma membrane of excitable cells:
1. the existence of a resting membrane potential
2. the presence of specific types of ion channels

Ion Channels
• When ion channels are open, they allow specific ions to move
across the plasma membrane, down their electrochemical
gradient a concentration (chemical) difference plus an electrical
difference.
• Positively charged cations move toward a negatively charged area,
and negatively charged anions move toward a positively charged.
• Ion channels open and close due to the presence of “gates.”
• The gate is a part of the channel protein that can seal the channel
pore shut or move aside to open the pore . The electrical signals
produced by neurons and muscle fibers rely on four types of ion
channels:.

1. LEAK CHANNELS :-
• The gates of leak channels randomly alternate between open and closed
positions. Typically, plasma membranes have many more potassium ion
(K+) leak channels than sodium ion (Na+) leak channels, and the
potassium ion leak channels are leakier than the sodium ion leak
channels.
• Leak channels are found in nearly all cells, including the dendrites, cell
bodies, and axons of all types of neurons.

2. A ligand-gated channel
• A ligand-gated channel opens and closes in response to the binding of
a ligand (chemical) stimulus. A wide variety of chemical ligands
including neurotransmitters, hormones, and particular ions—can
open or close ligand-gated channels.
• Ligand-gated channels are located in the dendrites of some sensory
neurons, such as pain receptors, and in dendrites and cell bodies of
interneurons and motor neurons.

3. A mechanically-gated channel
• A mechanically-gated channel opens or closes in response to mechanical
stimulation in the form of vibration (such as sound waves), touch, pressure,
or tissue stretching.
• Examples of mechanically-gated channels are those found in auditory
receptors in the ears, in receptors that monitor stretching of internal
organs, and in touch receptors and pressure receptors in the skin.

4. A voltage-gated channel
• Opens in response to a change in membrane potential (voltage).
• Voltage-gated channels participate in the generation and
conduction of action potentials in the axons of all types of
neurons.

Resting Membrane Potential
The resting membrane potential exists because of a small build-up of
negative ions in the cytosol along the inside of the membrane, and an
equal build-up of positive ions in the extracellular fluid (ECF) along the
outside surface of the membrane

• Such a separation of positive and negative electrical charges is a
form of potential energy, which is measured in volts or millivolts
(1 mV 0.001 V).
• The greater the difference in charge across the membrane, the
larger the membrane potential (voltage).
• In neurons, the resting membrane potential ranges from -40 to -
90 mV.
• A cell that exhibits a membrane potential is said to be polarized.
Most body cells are polarized; the membrane potential varies
from 5 mV to 100 mV in different types of cells.

Generation of Action Potentials
• An action potential (AP) or impulse is a sequence of rapidly
occurring events that decrease and reverse the membrane
potential and then eventually restore it to the resting state.
• An action potential has two main phases: a depolarizing phase
and a repolarizing phase

CONT…….
• During the depolarizing phase, the negative membrane potential
becomes less negative, reaches zero, and then becomes positive.
• During the repolarizing phase the membrane potential is restored
to the resting state of 70 mV.
• Following the repolarizing phase there may be an after-
hyperpolarizing phase, during which the membrane potential
temporarily becomes more negative than the resting level

CONT…..
• Two types of voltage-gated channels open and then close during an action
potential. These channels are present mainly in the axon plasma
membrane and axon terminals.
• The first channels that open, the voltage-gated Na channels, allow Na to
rush into the cell, which causes the depolarizing phase. Then voltage gated
K channels open, allowing K to flow out, which produces the repolarizing
phase.
• The after-hyperpolarizing phase occurs when the voltage-gated K channels
remain open after the repolarizing phase ends.

CONT….
• An action potential occurs in the membrane of the axon of a
neuron when depolarization reaches a certain level termed the
threshold (about 55 mV in many neurons).
• An action potential will not occur in response to a sub-threshold
stimulus, a weak depolarization that cannot bring the membrane
potential to threshold .
• A supra-threshold stimulus, a stimulus that is strong enough to
• depolarize the membrane above threshold.
• Each of the action potentials caused by a supra-threshold
stimulus has the same amplitude (size) as an action potential
caused by a threshold stimulus.

• The greater the stimulus strength above threshold, the greater the
frequency of the action potentials until a maximum frequency is
reached as determined by the absolute refractory period .
• An action potential either occurs completely or it does not occur at
all. This characteristic of an action potential is known as the all-or-
none principle

•Depolarizing Phase
• When a depolarizing graded potential or some other stimulus
causes the membrane of the axon to depolarize to threshold,
voltage-gated Nachannels open rapidly.
• Both the electrical and the chemical gradients favor inward
movement of Na, and the resulting inrush of Na causes the
depolarizing phase of the action potential.
• The inflow of Nachanges the membrane potential from 55 mV to
30 mV. At the peak of the action potential, the inside of the
membrane is 30 mV more positive than the outside.

• Repolarizing Phase
• Shortly after the activation gates of the voltage-gated Na
channels open, the inactivation gates close.
• Now the voltage-gated Nachannel is in an inactivated state. In
addition to opening voltage-gated Nachannels, a threshold level
depolarization also opens voltage-gated Kchannels.
• Because the voltage-gated Kchannels open more slowly, their
opening occurs at about the same time the voltage-gated Na
channels are closing.
• The slower opening of voltage-gated Kchannels and the closing
of previously open voltage-gated Nachannels produce the
repolarizing phase of the action potential

• Slowing of Na in flow and acceleration of Koutflow cause the
membrane potential to change from 30 mV to 70 mV.
• Repolarization also allows inactivated Nachannels to revert to
the resting state.

• After-hyperpolarizing Phase
• While the voltage-gated Kchannels are open, outflow of K may
be large enough to cause an after-hyperpolarizing phase of the
action potential
• During this phase, the voltage-gated Kchannels remain open
and the membrane potential becomes even more negative
(about -90 mV).

Refractory Period
• The period of time after an action potential begins during which an
excitable cell cannot generate another action potential in response to
a normal threshold stimulus is called the refractory period.
• During the absolute refractory period, even a very strong stimulus
cannot initiate a second action potential. This period coincides with
the period of Na + channel activation and inactivation.
• The relative refractory period is the period of time during which a
second action potential can be initiated, but only by a larger than
normal stimulus.
• It coincides with the period when the voltage-gated Kchannels are still
open after inactivated Nachannels have returned to their resting state

Propagation of Action Potentials
• To communicate information from one part of the body to
another, action potentials in a neuron must travel from where
they arise at the trigger zone of the axon to the axon terminals.
• In contrast to the graded potential, an action potential is not
decremental (it does not die out).
• Instead, an action potential keeps its strength as it spreads
along the membrane. This mode of conduction is called
propagation .

Cont…..
• In a neuron, an action potential can propagate in this direction
only it cannot propagate back toward the cell body because any
region of membrane that has just undergone an action potential
is temporarily in the absolute refractory period and cannot
generate another action potential.
• Because they can travel along a membrane without dying out,
action potentials function in communication over long distances.

Continuous and Saltatory Conduction
• There are two types of propagation:
1. continuous conduction
2. saltatory conduction.
• The type of action potential propagation described so far is
continuous conduction, which involves step-by step
depolarization and repolarization of each adjacent segment of
the plasma membrane.

Cont…
• In continuous conduction, ions flow through their voltage-gated
channels in each adjacent segment of the membrane. Note that
the action potential propagates only a relatively short distance
in a few milliseconds.
• Continuous conduction occurs in unmyelinated axons and in
muscle fibers.

2. Saltatory conduction saltat- leaping),
• The special mode of action potential propagation that occurs
along myelinated axons, occurs because of the uneven
distribution of voltage- gated channels.
• Few voltage-gated channels are present in regions where a
myelin sheath covers the axolemma. By contrast, at the nodes
of Ranvier (where there is no myelin sheath), the axolemma has
many voltage-gated channels.
• Hence, current carried by Na and Kflows across the membrane
mainly at the nodes.

Cerebrospinal Fluid
• Cerebrospinal fluid (CSF) is a clear, colorless liquid composed
primarily of water that protects the brain and spinal cord from
chemical and physical injuries.
• It also carries small amounts of oxygen, glucose, and other
needed chemicals from the blood to neurons and neuroglia.
• CSF continuously circulates through cavities in the brain and
spinal cord and around the brain and spinal cord in the
subarachnoid space (the space between the arachnoid mater
and pia mater).
• The total volume of CSF is 80 to 150 mL (3 to 5 oz) in an adult.
CSF contains small amounts of glucose, proteins, lactic acid,
urea, cations (Na, K, Ca2, Mg2), and anions (Cl– and HCO3–), it
also contains some white blood cells.

Functions of CSF
• The CSF has three basic functions:
1. Mechanical protection. CSF serves as a shock-absorbing
medium that protects the delicate tissues of the brain and spinal
cord from jolts that would otherwise cause them to hit the bony
walls of the cranial cavity and vertebral canal. The fluid also
buoys the brain so that it “floats” in the cranial cavity.
2. Homeostatic function. The pH of the CSF affects pulmonary
ventilation and cerebral blood flow, which is important in
maintaining homeostatic controls for brain tissue. CSF also
serves as a transport system for polypeptide hormones secreted
by hypothalamic neurons that act at remote sites in the brain.
3. Circulation. CSF is a medium for minor exchange of nutrients
and waste products between the blood and adjacent nervous
tissue.

Spinal Cord Physiology
• The spinal cord has two principal functions in maintaining
homeostasis:
1. nerve impulse propagation
2. integration of information.
• The white matter tracts in the spinal cord are highways for
nerve impulse propagation. Sensory input travels along these
tracts toward the brain, and motor output travels from the brain
along these tracts toward skeletal muscles and other effector
tissues.
• The gray matter of the spinal cord receives and integrates
incoming and outgoing information.

Sensory and Motor Tracts
• As noted previously, one of the ways the spinal cord promotes
homeostasis is by conducting nerve impulses along tracts.
Often, the name of a tract indicates its position in the white
matter and where it begins and ends.
• The anterior cortico-spinal tract conveys nerve impulses from
the brain toward the spinal cord, it is a motor (descending) tract.
• Nerve impulses from sensory receptors propagate up the spinal
cord to the brain along two main routes on each side:
1. the spino-thalamic tract
2. the posterior column.

Spinothalamic tract
• The spinothalamic tract conveys nerve impulses for sensing
pain, warmth, coolness, itching, tickling, deep pressure, and
crude touch.

Posterior column
• The posterior column consists of two tracts:
a. the gracile fasciculus
b. the cuneate fasciculus
The posterior column tracts convey nerve impulses for discriminative
touch, light pressure, vibration, and conscious proprioception (the
awareness of the positions and movements of muscles, tendons, and
joints).

• The sensory systems keep the CNS informed of changes in the
external and internal environments. The sensory information is
integrated (processed) by interneurons in the spinal cord and
brain.
• Responses to the integrative decisions are brought about by motor
activities (muscular contractions and glandular secretions). The
cerebral cortex, the outer part of the brain, plays a major role in
controlling precise voluntary muscular movements. Other brain
regions provide important integration for regulation of automatic
movements.

• Motor output to skeletal muscles travels down the spinal
cord in two types of descending pathways: direct and
indirect.
• The direct motor pathways include the lateral corticospinal,
anterior corticospinal, and corticobulbar tracts.
• They convey nerve impulses that originate in the cerebral
cortex and are destined to cause voluntary movements of
skeletal muscles.
• Indirect motor pathways include the rubrospinal ,
tectospinal vestibulospinal, lateral reticulospinal, and
medial reticulospinal tracts.

• These tracts convey nerve impulses from the brain stem to
cause automatic movements and help coordinate body
movements with visual stimuli.
• Indirect pathways also maintain skeletal muscle tone, sustain
contraction of postural muscles, and play a major role in
equilibrium by regulating muscle tone in response to
movements of the head.

Reflexes and Reflex Arcs
• The second way the spinal cord promotes homeostasis is by
serving as an integrating center for some reflexes.
• A reflex is a fast, involuntary, unplanned sequence of actions
that occurs in response to a particular stimulus.
• Some reflexes are inborn, such as pulling your hand away from
a hot surface before you even feel that it is hot. Other reflexes
are learned or acquired.

• When integration takes place in the spinal cord gray matter,
the reflex is a spinal reflex. An example is the familiar patellar
reflex (knee jerk).
• If integration occurs in the brain stem rather than the spinal
cord, the reflex is called a cranial reflex. An example is the
tracking movements of your eyes as you read this sentence.
• You are probably most aware of somatic reflexes, which
involve contraction of skeletal muscles.
• Equally important, however, are the autonomic (visceral)
reflexes, which generally are not consciously perceived. They
involve responses of smooth muscle, cardiac muscle, and
glands.

• Nerve impulses propagating into, through, and out of the CNS
follow specific pathways, depending on the kind of information,
its origin, and its destination.
• The pathway followed by nerve impulses that produce a reflex
is a reflex arc (reflex circuit). A reflexarc includes the following
five functional components

1. Sensory receptor.
• The distal end of a sensory neuron (dendrite) or an associated
sensory structure serves as a sensory. receptor.
• It responds to a specific stimulus a change in the internal or
external environment by producing a graded potential called a
generator (or receptor) potential.
• If a generator potential reaches the threshold level of
depolarization, it will trigger one or more nerve impulses in the
sensory neuron.

2. Sensory neuron. The nerve impulses propagate from the
sensory receptor along the axon of the sensory neuron to the
axon terminals, which are located in the gray matter of the
spinal cord or brain stem. From here, relay neurons send
nerve impulses to the area of the brain that allows conscious
awareness that the reflex has occurred.

• 3. Integrating center.
• One or more regions of gray matterwithin the CNS acts as an
integrating center. In the simplest type of reflex, the integrating
center is a single synapse between a sensory neuron and a
motor neuron.
• A reflex pathway having only one synapse in the CNS is termed
a monosynaptic reflex arc.
• More often, the integrating center consists of one or more
interneurons, which may relay impulses to other interneurons as
well as to a motor neuron.
• A polysynaptic reflex arc (poly- many) involves more than
two types of neurons and more than one CNS synapse.

4. Motor neuron.
• Impulses triggered by the integrating center propagate out of
the CNS along a motor neuron to the part of the body that will
respond.
5. Effector.
The part of the body that responds to the motor nerve impulse,
such as a muscle or gland, is the effector. Its action is called a
reflex. If the effector is skeletal muscle, the reflex is a somatic
reflex. If the effector is smooth muscle, cardiac muscle, or a
gland, the reflex is an autonomic (visceral) reflex.

Functional Organization of the Cerebral
Cortex
• Specific types of sensory, motor, and integrative signals are
processed in certain regions of the cerebral cortex .
• Generally, sensory areas receive sensory information and are
involved in perception, the conscious awareness of a sensation;
• motor areas control the execution of voluntary movements; and
• association areas deal with more complex integrative functions
such as memory, emotions, reasoning, will, judgment, personality
traits, and intelligence.
• In this section we will also discuss hemispheric lateralization and
brain waves.

Sensory Areas
• Sensory impulses arrive mainly in the posterior half of both
cerebral hemispheres, in regions behind the central sulci. In the
cerebral cortex, primary sensory areas receive sensory
information that has been relayed from peripheral sensory
receptors through lower regions of the brain.

• They usually receive input both from the primary areas and from
other brain regions.
• Sensory association areas integrate sensory experiences to
generate meaningful patterns of recognition and awareness.
For example, a person with damage in the primary visual area
would be blind in at least part of his visual field, but a person
with damage to a visual association area might see normally yet
be unable to recognize ordinary objects such as a lamp or a
toothbrush just by looking at them.

The following are some important
sensory areas
• The primary somatosensory area is located directly posterior
to the central sulcus of each cerebral hemisphere in the
postcentral gyrus of each parietal lobe.
• The primary somatosensory area receives nerve impulses for
touch, pressure, vibration, itch, tickle, temperature (coldness
and warmth), pain, and proprioception (joint and muscle
position) and is involved in the perception of these somatic
sensations.
• A “map” of the entire body is present in the primary
somatosensory area:

• The primary visual area, located at the posterior tip
of the occipital lobe mainly on the medial surface (next to the
longitudinal fissure), receives visual information and is involved
in visual perception.
• The primary auditory area , located in the superior part of the
temporal lobe near the lateral cerebral sulcus, receives
information for sound and is involved in auditory perception.

• The primary gustatory area , located at the base of
the postcentral gyrus superior to the lateral cerebral sulcus in
the parietal cortex, receives impulses for taste and is involved
in gustatory perception and taste discrimination.
The primary olfactory area, located in the temporal
lobe on the medial aspect (and thus not visible in, receives
impulses for smell and is involved in olfactory perception.

Motor Areas
• Motor output from the cerebral cortex flows mainly from the
anterior part of each hemisphere. Among the most important
motor areas are the following :-
1. The primary motor area is located in the precentral gyrus of
the frontal lobe. As is true for the primary somatosensory
area, a “map” of the entire body is present in the primary
motor area: Each region within the area controls voluntary
contractions of specific muscles or groups of muscles

• Electrical stimulation of any point in the primary motor area
causes contraction of specific skeletal muscle fibers on the
opposite side of the body.
• Different muscles are represented unequally in the primary
motor area. More cortical area is devoted to those muscles
involved in skilled, complex, or delicate movement.

2. Broca’s speech area
• It is located in the frontal lobe close to the lateral cerebral sulcus.
• Speaking and understanding language are complex activities
that involve several sensory, association, and motor areas of the
cortex.
• In about 97% of the population, these language areas are
localized in the left hemisphere. The planning and production of
speech occur in the left frontal lobe in most people.
• From Broca’s speech area, nerve impulses pass to the premotor
regions that control the muscles of the larynx, pharynx, and
mouth

Association Areas
• The association areas of the cerebrum consist of large areas of
the occipital, parietal, and temporal lobes and of the frontal
lobes anterior to the motor areas.
• Association areas are connected with one another by
association tracts and include the following

a. Somatosensory association area is just
• posterior to and receives input from the primary somatosensory
area, as well as from the thalamus and other parts of the brain.
• This area permits you to determine the exact shape and texture
of an object by feeling it, to determine the orientation of one
object with respect to another as they are felt, and to sense the
relationship of one body part to another.

• Another role of the somatosensory association area is the
storage of memories of past somatic sensory experiences,
enabling you to compare current sensations with previous
experiences.
• For example, the somatosensory association area allows you to
recognize objects such as a pencil and a paperclip simply by
touching them.

b. visual association area located in the
• occipital lobe, receives sensory impulses from the primary visual
area and the thalamus.
• It relates present and past visual experiences and is essential for
recognizing and evaluating what is seen.
• For example, the visual association area allows you to recognize
an object such as a spoon simply by looking at it.

c. facial recognition area, corresponding roughly to areas in the
inferior temporal lobe, receives nerve impulses from the visual
association area.
• This area stores information about faces, and it allows you to
recognize people by their faces.
• The facial recognition area in the right hemisphere is usually
more dominant than the corresponding region in the left
hemisphere

d. auditory association area
located inferior and posterior to the primary auditory area in the
temporal cortex, allows you to recognize a particular sound as
speech, music, or noise.
e. orbitofrontal cortex, corresponding roughly to along the
lateral part of the frontal lobe, receives sensory impulses from the
primary olfactory area.
This area allows you to identify odors and to discriminate among
different odors.

f. Wernicke’s area (posterior language area),
a broad region in the left temporal and parietal lobes, interprets
the meaning of speech by recognizing spoken words. It is active
as you translate words into thoughts.
g. common integrative area is bordered by somatosensory,
visual, and auditory association areas. It receives nerve impulses
from these areas and from the primary gustatory area, the
primary olfactory area, the thalamus, and parts of the brain stem.

Nervous system

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